During recent years, many efforts to increase the sampling rate and the effective number of bit (ENOB) of analogue-to-digital converters (ADC) have been made. Analogue-to-digital converters are required for the conversion of analogue into digital signals. These analogue signals can be wireless, like radar signals, they can be electrical, or optical signals. The network nodes of the internet for instance, require extensive analogue-to-digital conversion of coherent signals. The most common sampling method is a sampling with electronically sample-and-hold circuits. Since the data rates in the worldwide networks increase every year, these circuits come to their limits, which is expressed in a drastically increase of the energy consumption.
For an ideal sampling the signal-to-sample is multiplied by a Dirac-Delta pulse sequence. As a result, every single pulse of the sequence is weighted with the value of the signal-to-sample at this distinct time. This amplitude value can correspond to a voltage, a current, an electrical or magnetical field or any other measuring value. However, a Dirac-Delta sequence is just a mathematical construct. Due to causality, it cannot be generated in practice.
Electronically sample-and-hold circuits open a gate for a distinct time and take the value from the signal-to-sample during the time of the opening of the gate. Electronically samplers can only process electrical signals (current, voltage). Optical signals have to be transformed to the electrical domain by a photodiode or a balanced photo-detector, for instance. The sampled value will be further processed by the following electronics to form a complete analog-to-digital converter. The opening of the gate can be approximated by a rectangular function. Thus contrary to an ideal sampling, the signal-to-sample is multiplied by a rectangular pulse sequence. This leads to a distortion of the sampled signal, which has to be compensated by energy consuming electronics. The higher the sampling rate, the higher is the energy consumption of electronic analog-to-digital converters. Additionally, the analogue bandwidth of electronic samplers is restricted to a few 10th of GHz.
In contrast, optical sampling methods can sample very high analogue bandwidths. In an optical sampler the optical signal-to-sample is multiplied by a pulse sequence, generated by an optical pulse source, typically a mode-locked laser. For the multiplication a nonlinear effect like four wave mixing or cross phase modulation in a nonlinear crystal or a nonlinear fiber can be used. Another possibility is the multiplication between the signal-to-sample and the pulse source in a modulator or a balanced-photodetector, or -receiver. However the nonlinearities in the modulator, the balanced-receiver or the nonlinear element lead to a distortion of the sampled signal. At the same time, mode-locked lasers are quite expensive and they hinder an integration of optical sampling on a chip together with the required electronics in order to build a complete analog-to-digital converter. For the multiplication between the signal-to-sample and the pulse source in a modulator or a balanced receiver, the analogue bandwidth of the signal-to-sample is restricted by the bandwidth of the modulator or balanced receiver.